In addition to being much cheaper than bar clamps, pipe clamps allow for much higher clamping pressure. According to this article in Fine Woodworking, a typical parallel clamp can reach about 370 pounds of pressure. 3/4" pipe clamps can reach 1,050 pounds and the I beam style bar clamps can reach 1,350 pounds.
One of the downsides to pipe clamps was that they could be difficult to keep aligned when on the underside of a glue up. A lot of those problems can be eliminated by use of newer style clamps such as the Rockler Sure Foot clamps